1. Field of the Invention
The present invention relates to an electrical interconnection and means of making the interconnections, which are useful in electronic packaging applications such as in semiconductor integrated circuit chips and circuit boards and cards, cables and modules.
Recent developments in integrated circuits have clearly demonstrated the benefits which can be achieved by fabricating electrical devices into smaller and smaller packages. These small packages are densely packed, being multilevel with signal and power planes and other features on various levels and means of interconnecting selected levels to one another. The interconnections themselves provide sites for potential signal degradation. For example, interconnections between levels of conductor lines, and between conductor lines of a printed circuit board (PCB) or card and any electrical devices mounted thereon can be made between conductive areas called pads. Impedance matching, minimum number of discontinuities and redundancy must be present at these interconnections in order to permit rapid, low noise, low loss, low resistance signal transmission. Approaches used at present in devising interconnections for these electronic packages may require numerous processing steps, and the attachment of surface mounted devices may require soldering and rework, often involving exposing the components to destructive temperature cycling.
Copending application Ser. No. 07/520,335, filed May 7, 1990 to Burns et al., now U.S. Pat. No. 5,118,299 and commonly assigned with the present invention describes a single and a double sided contact comprised of polymeric cones which have been formed in polymer sheets by excimer laser and have been surface metallized. The cone connectors of the present invention, like that in the copending application, provide the low cost, high frequency, redundant, fine tipped high performance pad-on-pad contacts required. However, the cones of the present invention, being differently comprised and differently fabricated, provide an alternative to those of the copending application.
2. Description of the Art
Electrical interconnections comprised of interdigitated dendritic projections are a fertile field of scientific inquiry. The conical projections of the present invention are distinguished from dendritic projections by the method of making, by composition, and by the controlled location and dimensions of conical projections. Schemes proposed to strengthen dendrites, such as coating with soft metal are described in IBM Technical Disclosure Bulletin, Vol. 22, No. 7, p. 2706 by Babuka et al and copending application Ser. No. 07/415,435 to Cuomo et al, filed Sep. 28, 1989 and commonly assigned with the present invention.
IBM Technical Disclosure Bulletin Vol. 22, No. 7, p. 2706, published December, 1979 by Babuka et al. describes a high density pad-to-pad connector on which dendrites are grown on a pad and coated with a liquid gallium alloy. When the dendritic pad is mated, the dendrites pierce the tarnished liquid metal film of a second pad and make the electrical contact.
IBM Technical Disclosure Bulletin, Vol. 24, No. 1A, June, 1981, p. 2, "Process For Producing Palladium Structures" by Armstrong et al describes that the small cross-section of the base of the dendrite is at least partly responsible for breakage of dendrites. It also describes the need for "wipe" to make low resistance contact, but states that the roughness of the dendritic surfaces provides sufficient wipe.
IBM Technical Disclosure Bulletin, Vol. 23, No. 8, January, 1981, p. 1, "Dendrite Connector System With Reinforced Base" by Armstrong agrees with the above diagnosis, but differs in the proposed cure, proposing instead reflowing tin around the bases of the dendrites. Dendrites as pad-to-pad contact elements are also described in Research Disclosure, March, 1988, No. 287, p. 28748, "Method to Provide Multiple Dendritic Contact Points for High Density Flat on Flat Connector System", disclosed anonymously. Again, the dendrites, described are irregularly shaped and randomly located. However, the reduced connector length of the dendrites are described as providing noise reduction and improved signal speed, and the references suggests that having multiple contact points lowers contact resistance.
The cones of the present invention, unlike the dendrites of several of the above references, do not require reinforcement.
Other means in the art of making electrical interconnection between contact pads include spheres (U.S. Pat. No. 3,634,807 issued Jan. 11, 1972 to Grobe et al, U.S. Pat. No. 4,604,644 issued Aug. 5, 1986 to Beckham et al) conductive rods (U.S. Pat. No. 4,644,130 issued Feb. 17, 1987 to Bachmann, U.S. Pat. No. 4,050,756 issued Sep. 27, 1977 to Moore, and U.S. Pat. No. 4,240,198, issued Dec. 23, 1980 to Alonso), hollow posts (U.S. Pat. No. 3,725,845) and third structures interposed between and parallel to the connector pads but separate from both (U.S. Pat. Nos. 3,881,799, issued May 6, 1975 to Elliott et al and 3,634,807, issued Jan. 11, 1972 to Grobe et al).
Flat-topped protrusions, permanently connecting pads between levels in a multilayer structure are described in the art (U.S. Pat. No. 4,751,563, issued Jun. 14, 1988 to Laibowitz et al).
U.S. Pat. No. 3,634,807, issued Jan. 11, 1972 to Grobe et al. describes a removably attachable contact comprising a plurality of hollow metal spheres or wire balls mounted in a predetermined pattern on either side of a flexible insulating sheet. Alternatively, metal is deposited in openings at the intersection of thin strips of insulating material. In another embodiment, a conductive sheet is sandwiched between sets of contact elements. These embodiments are designed to be relatively inflexible in the X-Y direction and flexible in the Z direction.
U.S. Pat. No. 3,725,845 issued Apr. 3, 1975 to Moulin describes a hermaphroditic connector comprising a plurality of hollow posts. It is a large scale connector for watertight use with cables in geophysical surveying, rather than for use with microminiature contact pads in packaging.
U.S. Pat. No. 3,881,799 issued May 6, 1975 to Elliott et al. describes a connector that comprises a plurality of domes projecting from both sides of a spring matrix, interposing a third element between the contacts to be connected, the third element being integral to neither.
All the above nondendritic contact means are inadequate for use in high packing density structures, being of dimensions which are too large and too vulnerable to dirt contamination.
U.S. Pat. No. 4,644,130 issued Feb. 17, 1987 to Bachmann describes a plurality of elastomeric connector rods which have been rendered conductive by being filled with conductive particles dispersed therein.
U.S. Pat. No. 4,751,563 issued Jun. 14, 1988 to Laibowitz et al. describes a method of making a cone shaped structure, having a carbonaceous surface contaminant, using an electron beam. A conductive layer is deposited on at least a portion of the cone and over the substrate area around the base of the cone. Then an insulating material is applied overall and any further processing is performed. Structures described in this patent are in the nature of through-holes, buried irreversibly within a unitary multilayer structure rather than being removably attached. Since electron beam radiation is used, the material from which the cone is comprised must of course be removable by electron beams.
Unlike connectors described in the art, the electrochemically machined (ECM) connector of the present invention is simply fabricated, reproducible, completely metallic, substantially smooth, of a single preselected height, non-brittle and applicable to interconnection of high density circuitry. Furthermore, the neutral salt electrolyte solutions in which the cones of the present invention are prepared pose no known safety problem, even when recirculated as in the present invention.
A number of fundamental studies of electrochemical machining have been reported in journal literature. The importance of mass transport conditions for high rate dissolution of iron and nickel in 5M NaCl, 5M NaClO3 and 6M NaNO3 was discussed in "On the Role of Mass Transport in High Rate Dissolution of Iron and Nickel in ECM Electrolytes" Parts I and II, by Datta et al. in Electrochimica Acta, Vol. 25, pages 1255-1263, 1980. Anodic levelling of nickel peaks in NaCl solution was discussed in "On the Theory of Anodic Levelling: Model Experiments with Triangular Nickel Profiles in Chloride Solution" by Clerc et al. in Electrochimica Acta, Vol. 29, pages 1477-1486, 1984. More recently, micromachining of small dimensions ranging from several microns to millimeters has been discussed for various metal and semiconductor-electrolyte systems in "Application of Chemical and Electrochemical Micromachining in the Electronics Industry", by Datta et al. in Journal of the Electrochemical Society, Vol. 136, No. 6, pages 285C-292C, June, 1989. Drilling of holes and slots in nickel and steel in neutral salt solutions is discussed in "Jet and Laser-Jet Electrochemical Machining of Nickel and Steel" by Datta et al. in Journal of the Electrochemical Society, Vol. 138, No. 8, pages 2251-2256, Aug. 1989. The feasibility of etching grooves in stainless steel in neutral salt solutions through photoresist masks, and problems inherent in the process were discussed in "Electrochemical Dissolution of Stainless Steels in Flow Channel Cells With and Without Photoresist Masks" by Rosset et al., Journal of Applied Electrochemistry, Vol. 20, pages 69-76, 1990.
These articles in general report on fundamental studies of electrochemical machining which have been performed under controlled hydrodynamic conditions in order to acquire an understanding of the anodic behavior of the metal-electrolyte system. Electrochemical machining involves a high rate of metal removal from a workpiece that has been made anodic in an electrolytic cell. In neutral salt solution, hydrogen evolution takes place at the cathode. The process of metal removal from the anode, being electrochemical in nature, is independent of the hardness of the metal to be removed. Unlike mechanical machining, the ECM process does not introduce stresses into the machined workpiece.
Based on electrochemical studies, ECM systems are divided into two types: passivating and non-passivating. Passivating electrolytes contain oxidizing anions, such as nitrate anions, resulting in the formation of an oxide film on the anode material and possible oxygen evolution rather than metal dissolution at the anode at low current densities. At high current densities, however, a high rate of metal dissolution is possible in oxidizing systems. In non-passivating electrolytes, because of the presence of aggressive anions, oxide films do not form and oxygen evolution is not possible. Metal dissolution is the only anodic reaction in the non-passivating electrolyte. In conventional ECM, in which photolithographic masking is not used, the passivating electrolytes are generally preferred because of their inert nature and their ability to evolve oxygen at low current densities, thereby minimizing stray cutting effects. However, for the present invention, in which masking is employed, oxygen evolution may cause lifting or may otherwise damage the mask.
In both passivating and non-passivating systems, the rate of metal dissolution is dependent on the current density (or on the applied voltage), electrolyte concentration, and to an extent on the hydrodynamic conditions.
None of the journal articles summarized above describes the fine tipped pad-to-pad cone connectors of the present invention nor the manner of making.
Thus, it is one object of the present invention to use an electrochemical machining technique in a simple salt solution to provide high performance solid metal cone connectors.
It is a further object of the invention to provide the capability to interconnect high density packages of electrically mounted devices and PCBs and/or cards to each other and to cables.
It is a further object of the invention to provide an electrical interconnection which permits reliable, rapid, dirt tolerant, low noise, low loss, low resistance signal transmission.
It is a further object of the invention to provide a method of making the electrical interconnection described above in an efficient and controllable manner.
It is a further object of this invention to provide a conical electrical interconnection useful in the art of electronic packaging.
It is a further object of the invention to provide a low resistance electrical interconnection nondestructively connectable and disconnectable. consisting of solid metal cones.
Still another object of the invention is to provide a fabrication method to produce an electrical interconnection between two contact surfaces, at least one of which comprises essentially perpendicular conical projections of predetermined pattern and dimensions.
These and other objects, features and advantages of the present invention will become more apparent from the descriptions to follow.